FIG. 1 is a partial cross section illustrating an exemplary prior art electric device that comprises a component part 101 and an outer cover, which in this case consists of a lower cover part 102 and an upper cover part 103. Wired connections 104 to and from the electric appliance can be made through holes in the outer cover. We assume that among the components of the component part 101 is an electrolytic capacitor 105.
FIG. 2 illustrates another exemplary prior art device, which is inmoulded. This means that instead of any separately assembled outer cover the electric appliance comprises a moulded part 201, which encloses the component part 101. The moulded part 201 serves several purposes simultaneously. It acts as a mechanical support both to the components of the component part 101 and to possible connections 202, 203 to and from the component part 101. The moulded part 201 also balances temperature differences within the component part 101 by conducting heat away from hot components, and its inner parts constitute electrically isolating walls between the electrically conducting parts of the component part 101. In most cases it also isolates the component part 101 from the environment, keeping out water and dust. The outer surface of the moulded part 201 constitutes the outer surface of the whole electric appliance, which means that it must exhibit sufficient visual quality and give a pleasant tactile feeling. Commonly used materials for the moulded part 201 are thermosetting or thermoplastic hard mouldable polymers.
In addition to inmoulding, also potting or casting of electric appliances is known. As a difference to an inmoulded solution, potting or casting means that the appliance has an outer cover but the space inside it otherwise left free is filled with an epoxy resin or other casting material that flows in to fill all free space and hardens, so that the final result is a solid piece of material in which the space between the inside of the outer cover and the electronic components is filled with the hardened potting material.
A problem of an inmoulded or potted electric appliance is its vulnerability to explosion under certain very improbable but still possible circumstances. If an electrolytic capacitor 105 is subjected to overvoltage or a voltage of incorrect polarity, it may heat up so much that the electrolyte inside thereof begins to vaporize. Electric protection mechanisms such as fuses, overvoltage protection circuits and temperature- or voltage-triggered protective circuit elements are known; also the electrolytic capacitor itself may comprise internal conductor rupturing mechanisms or other means that react to exceptional conditions by electrically disconnecting the capacitor from the rest of the circuitry in the component part. However, the known protective measures may be so slow to react that a significant amount of vaporized electrolyte is still formed, causing a sudden rise of internal pressure within the moulded part. If the pressure difference between the inside and outside of the moulded or potted part becomes sufficiently large, the moulded part may crack.
The vulnerability to explosion of inmoulded or potted electric appliances becomes worse if the appliance should be made as small as possible. For example in the technical field of battery chargers of mobile electric devices there is a trend of squeezing the electronics of the charger device to an increasingly smaller space, so that the main body of a charger might be little larger than the wall plug of a power cord. Small size means thin material thickness of the moulded or potted material between the component part inside it and the outside, which in turn may lead to weaker mechanical strength and consequently to vulnerability to explosion even with relatively moderate pressure differences.
An obvious solution would be to increase the mechanical strength of the material used for the moulded or potted part, for example by mixing some reinforcing substance like glass fibres or carbon fibres to the basic polymer. However, using a reinforcing substance increases material costs and may cause disadvantages in other aspects of the manufacturing process, like increasing the mechanical wear of moulds, causing interference in the flow of the moulding material or producing unwanted visible effects on outer surfaces.
Responsible manufacturers of electric appliances test their products extensively during and immediately after the manufacturing process. Errors like an electrolytic capacitor assembled with incorrect polarity should reveal themselves in these tests. However, it is impossible to make the tests so extensive that they would take into account each and every form of exceptional operational conditions. It is also possible that a testing cycle goes so fast that an electrolytic capacitor with an internal fault does not make it to exploding before the test voltage is removed.
A prior art publication JP 8069948 discloses a protective arrangement in which the moulded cover of a capacitor element comprises a vent, through which the molten metal of a protective fuse may escape. Elastic material that filled the vent under normal circumstances gets pushed outwards by the molten metal and remains as a protective, isolating cover on top of the solidified metal after the protective device has operated. This solution does not solve the problem of expanding gaseous electrolyte. Apparently the author has assumed that the protective fuse is fast enough to cut off the electric current before any pressure buildup occurs. If the electrolyte would vaporize in the solution of JP 8069948, it might blow a way for itself through the vent, spraying the remnants of the elastic material and molten metal from the protective fuse all around.